The present invention relates to an exhaust gas purification apparatus for a diesel engine.
Diesel engines may be mounted on automobiles or other machines (e.g., construction machinery, farming machinery, and transportation machinery). A diesel particulate filter (DPF) may be arranged in an exhaust gas passage of a diesel engine. Particulate matter (PM) is accumulated in the DPF. A regeneration device injects fuel into the exhaust gas passage and burns the PM to regenerate the DPF. Japanese Laid-Open Patent Publication No. 2005-299438 describes an example of such a regeneration device that performs two types of regenerations. The first type is automatic travel regeneration, which automatically performs regeneration while the vehicle is traveling. The second type is manual regeneration, which is performed when a manual regeneration switch is operated while the vehicle is stopped. The regeneration of the DPF is controlled by switching between the two types of regeneration in accordance with a dilution state of oil.
More specifically, the regeneration device first obtains a regeneration occurrence coefficient Rc, which takes into account the manual regeneration occurrence, the automatic travel regeneration occurrence, and the travel distance and is obtained from the expression of (automatic travel regeneration occurrence×constant+manual regeneration occurrence×constant−constant)/travel distance. The regeneration device then compares the regeneration occurrence coefficient Rc with a predetermined value Ro to determine whether dilution is advanced. When the regeneration occurrence coefficient Rc is greater than the predetermined value Ro, the regeneration device determines that dilution is advanced, and manual regeneration is performed. When the regeneration occurrence coefficient Rc is less than or equal to the predetermined value Ro, automatic regeneration is performed when the vehicle is traveling.
Oil dilution occurs when unburned fuel, which is post-injected to perform DPF regeneration, mixes with the oil in the engine. The control described above is executed to avoid such oil dilution as much as possible. The control also reduces the burden on the user if the frequency of manual regeneration were to increase.
Normally, in a vehicle including a regeneration device for a DPF, regeneration is automatically started when PM accumulates in the DPF and regeneration becomes necessary. However, transportation machinery, such as a forklift, may be used for just a short operating time, such as five to ten minutes. In such a case, the engine may be stopped before regeneration of the DPF is completed. When the regeneration is interrupted in such a manner, the next two procedures may be taken.
(1) Continue regeneration in the next operating period, and repeat regeneration until the DPF is completely regenerated.
(2) Perform normal engine operation until the accumulated PM exceeds a regeneration threshold value, and restart regeneration when the accumulated PM exceeds the threshold value.
However, procedures (1) and (2) would both increase the occurrence of regenerations.
An increase in the occurrence of regenerations may lead to the next problems as described below.
(a) As shown in FIG. 11, the time required for raising the temperature to a temperature sufficient to burn PM increases, and additional energy becomes necessary (fuel efficiency falls). In detail, FIG. 11 shows the transition of the PM accumulation amount and the amount of fuel used for regeneration while the vehicle is traveling or operating when interruptions take place and when such interruptions do not take place. A regeneration interruption increases the time required to raise the temperature of the exhaust gas to the temperature that burns PM and prolongs the regeneration time. This increase the amount of fuel used for regeneration and lowers fuel efficiency.
(b) The time required to raise the temperature increases. Thus, in a system that performs post injection for regeneration, the time during which post injection is performed becomes long, and oil dilution reaches its limit within a short period of time.